Indicator for the detection of manganese (ii) ions

ABSTRACT

Indicator for the detection of manganese (II) ions comprising an abosrbent support carrier impregnated with a substantially insoluble sulfate salt of an aromatic amine oxidizable by MnO2 to a colored oxidation product.

United States Patent [191' Biiumer et al.

Nov. 6, 1973 INDICATOR FOR THE DETECTION OF MANGANESE (ll) IONSInventors: Wilhelm Biiumer; Dieter Schmitt;

Alfred Stein, all of Darmstadt, Germany Merck Patent Gesellschaft mitbeschrankter Haftung, Darmstadt, Germany Filed: Mar. 9, 1972 Appl. No.:233,316

Assignee:

Foreign Application Priority Data Mar. 10, 1971 Germany P 21 11 366.6

US. Cl 23/230 R, 23/253 TP, 252/408 Int. Cl. G0ln 31/22 Field of Search23/230 R, 253 TP;

References Cited OTHER PUBLICATIONS Stratton et al., Anal. Chem. 4, No.1, Jan. 1932, p. 2.

Sastry et al., Anal. Chem. 34, No. 10, Sept. 1962, p. 1,302-l,303.

Prirr lary Examiner-Robert M. Reese Attorney-J. William Millen et al.

20 Claims, No Drawings INDICATOR FOR THE DETECTION OF MANGANESE (II)IONS BACKGROUND OF THE INVENTION This invention relates to an indicatorand process for the colorimetric determination of manganese (II) ionsand method for the production thereof.

In analytical chemistry, the use of identification reagents for thecolorimetric determination of certain metal ions has become increasinglyimportant. Test sticks for making semiquantitative determinations in asimple manner are particularly desirable.

Test papers are known by which managanese (II) ions can be detected.This test is-based on the color reaction rendered by certain aromaticamines during oxidation. In order to conduct the test, these test papersare immersed into the solution to be tested for magnanese ions, and theninto l-'2N sodium hydroxide solution, which'converts any manganese ionsin the test solution to manganese dioxide in situ on the test paper bythe effect of atmospheric oxygen. Then, the test paper is bathed indilute acetic acid, thus producing a blue to violet color, depending-onthe aromatic amine employed and the pressure or absence of manganeseions in the test solution. g 1

However, the above-described test papers have the serious disadvantagethat they assume a blue to brown color-during manufacture and packagingin daylight. Thus, semi-quantitative manganese determinations areimpossible with such a test paper. An additional disadvantage is that,upon immersionof the test paper into the test solution, a concentrationgradient is produced on the paper, which makes it impossible toassociate the thus-formed color with a standard color scale in asatisfactory manner. Furthermore, iron (III) ions, in particular,interfere because these ions enclose and/or add manganese (II) ionsduring the formation of iron (III) hydroxide. The field ofapplication'of the conventional Mn indicator is consequently quitelimited.

It has now been found that these disadvantages can be avoided by Mn.indicators of this invention,-which can be stored without colordevelopment. These indicators can also be used for semi-quantitativedetermination because with the same manganese contents, substantiallyidentical coloring is developed. It is also possible to effectsemiquantitative determination of solutions containing a low manganese(II) ion concentration but high iron (III) ion concentrations.Surprisingly, by means of the novel indicator of this invention,satisfatory linear correlation between manganese ion concentration andthus-produced depth of color development is achieved. Consequently, thenovel indicators are substantially superior to the known indicatingmethods. Moreover, the indicators can be used to detect manganese (III)ions which occur very rarely in the solutions being tested.

. SUMMARY OF THE INVENTION The indicators of the detection of manganese(II) ions of this invention comprise an absorbent support impregnatedwith a'substantially insoluble sulfate salt of an aromatic amineoxidizable by manganese dioxide to a colored oxidation product. In itsmethod of use aspect, this invention relates to a method for thedetection of Mn ions in a test solution employing an indicator of thisinvention. In its process aspect, this invention relates to theproduction of an indicator of this invention by the impregnation of anabsorbent support with a solution containing an aromatic amine asdefined herein, a water soluble metal sulfate and an organic Fe ioncomplexing agent.

DETAILED DISCUSSION Suitable absorbent supports or carriers includethose customarily employed for the production of indicator reagents.Filter paper is most commonly employed. However, other forms ofabsorbent cellulose and cellulose derivatives and absorbent products, e.g., polyester fiber and fiberglass laminate can likewise be used.

Suitable aromatic amines are those capable of forming a sulfate salt oflow-solubility and which is oxidizable by manganese dioxide. If theamine is colored, and it preferably is not, the oxidation product mustexhibit a color different from the free aromatic amine of the sulfatesalt thereof.

The exact structure of the aromatic amine is not critical so long as itssulfate salt is substantially insoluble in water and the amine isoxidizable by MnO under the test conditions, e.g., about roomtemperature after immersion in strong base followed by moderately strongacid, e.g., 10 percent aqueous acetic acid, and its oxidation product iscolored and the starting amine is colorless or a different color thanits oxidation product. Substantially insoluble as used herein means asolubility preferably less than 1 percent and more preferably less than0.2 percent in water at room termperature. The requirement that theoxidation product is colored is readily met because many aromatic aminesare readily oxidized to colored oxidation products.

Preferred aromatic amines are primary amines, especially thosecontaining at least two primary amine groups. The aromaticity ispreferably provided by a I benzene ring, more preferably at least twobenzene rings, each bearing a primary amine group, bonded or fusedtogether to form, e.g., a biphenyl, naphthalene, flurene, anthracene,etc. ring system. The aromatic a'rnine can hear one or more othersubstituents, e. g.,

I alkoxy or alkyl of one to four atoms, nitro, cyano, halo,

ticular o-tolidine, benzidine, o-dianisidine and 2,7-

e. g.,.Cl, Br, etc.

More particularlyamines of thhe general formula diaminofluorene.

Since manganese is very frequently associated with iron in the solutionsto be tested and the detection of .manganese is considerably impairedthereby, it is advantageous that the absorbent support or carrieradditionally is impregnated with a complex-forming agent which iscapable of complexing iron ions and other heavy metal ions, but notmanganese (II) ions and manganese (Ill) ions. Particularly advantageousare tartaric acid, citric acid and/or the salts thereof.

It is particularly advantageous to impregnate the absorbent supportsuccessively with at least two different solutions which in each casecontain only part of the above-indicated components. Ifdifferentsolutions are used for the impregnating step, it is necessaryto at least partially dry the absorbent support between the impregnationoperations so as to enable the absorption thereon of the secondimprenating solution;

A convenient method of uniformly impregnating the adsorbent carriersupport with the selected slightly soluble aromatic amine sulfate is tosuccessively impregnate, in either order, the carrier with an aqueous orsolvent solution of the free amine or a soluble salt thereof and with asolution of soluble metal sulfate, e.g., alkalimetal or alkaline earthsulfate. Preferably the aromatic amine solution is applied first andthen, preferably after drying the amine-impregnated support, a solutionof a soluble metal sulfate is added in molar proportion at leastsufficient to convert all of theamine to its amine sulfate salt andpreferably in a substantial excess. For this purpose, all water-solublealkali sulfates and/or alkaline earth sulfates are usable, preferablysodium, po tassium, and magnesium sulfate, particularly magnesiumsulfate.

It is also possible to impregnate the absorbent carrier with the aminesulfate by forming a suspension of the amine sulfate by mixing asolution of the amine or a soluble salt thereof with a solution of themetal sulfate and then impregnating the absorbent support with theresulting suspension. Furthermore, it is possible to add thelow-solubility sulfate of the aromatic amine to a pulp of paper fibersand then manufacture a test paper therefrom a conventional'manner on asheet-producing or a papermaking machine. This paper can thereafteroptionally be impregnated with an organic Fe complexforming agent.

Ordinarily, it is advantageous to employ two different impregnatingsolutions, by means of which the absorbent support is successivelyimpregnated, preferably first with the amine.

The concentration of the aromatic amine and metal sulfate in theimpregnating solution or solutions is not critical. For example, theimpregnating solution can contain the aromatic amine, metal sulfate and,optionally, the iron ion complexing agent in the followingconcentrations:

0.05 1 percent of aromatic amine,

0.05 4 percent of a water-soluble alkali sulfate and- /or alkaline earthsulfate, and preferably in at least one of the impregnating solutions,

0.2 5 percent of an organic complex-forming agent capable of formingcomplexes with iron ion-s.

For producing a first impregnating solution I, dissolve 0.05 l g. of thearomatic amine in 100 ml. of an organic aqueous organic or aqueoussolvent, e. g., dilute aqueous acetic acid. Suitable organic solventsinclude the lower alkanols, especially methanol, other readily volatileorganic solvents, e. g., keytones, ethers, esters and aliphatic andaromatic halohydrocarbons and hydrocarbons, in which the aromatic amineis soluble can also be used.

To prepare a second impregnating solution 11, dissolve 0.05 4 g. of awater-soluble metal sulfate, e.g., alkali sulfate and/or alkaline earthsulfate, optionally also 0.2 5 g. of the organic complexing agent in ml.of water.

The absorbent carriers, preferably filter paper, are successivelyimpregnated with the impregnating solution or solutions in aconventional manner so as to provide a pickup of the solution of about-300 percent, preferably about l00200 percent, calculated on the dryweight of the carrier. The impregnated papers can either be cut intohandy strips, or they can be processed into preferably square pieces.They can then be conventionally glued onto plastic films, paper strips,or metallic strips, or sealed onto or into such films and strips.

The indicators of the invention, as they are used for the colorimetricdetermination of manganese ions, comprise an absorbent carrier uniformlyimpregnated (per in of surface area) with 0.1-l0g; preferably 0.5-3 g.,of the aromatic amine sulfate. Ordinarily, unless it is removed bypost-washing of the novel impregnated carrier, there will also beadsorbed thereon other reaction products resulting from the exchangereaction of the-amine (or a soluble salt thereof) and the soluble metalsulfate, e.g., sodium or magnesium acetate. Preferably, there is alsoimpregnated therein'from one-fifth to 100 times by weight, calculated onthe amine sulfate, of an iron ion organic-complexing agent.

To detect manganese ions, one of the abovementioned strips is immersedinto the solution to be tested, until the testing zone of the test stripis fully wetted. Then, the testing zone is first briefly bathed, e.g.,about 15 seconds, in a strongly alkaline solution, e.g., l2N sodiumhydroxide, to raise the pH to about 14 or higher. During this step, anymanganese (II) hydroxide which is formed is further oxidized tomanganese dioxide by the atmospheric oxygen present. Thereafter, thetesting zone is bathed for about '15 seconds in dilute acid, e.g., 10percent strength acetic acid, to lower the pH to an acid pH. In thisacidic medium, the aromatic amine is oxidized by the thus-formedmanganese dioxide to the corresponding dyestuff. When manganese ions arepresent in the solution to be tested, the testing zone is then coloredusually blue, the exact color depending on the selected aromatic amine.This coloring is compared with a standard color scale obtained on usingsolutions of varying known Mn concentrations. The manganese ionconcentration of the test solution can readily be determined by colorcomparision with the colors developed with the known Mn solutions. Inthis semiquantitative manner, distinct shading of the blue coloring canbe perceived between lo, 25, 50, 100, 250 and 500 mg./l. (or p.p.m.) ofmanganese ions. The novel indicator is also distinguished by a very highselectivity and sensitivity. It is possible to detect with certainty thepresence of as little as 10 mg./l. of manganese ions by the bluecoloring which develops in the reagent zone of the test strip.

In the pH range of l7, the accuracy of detection is independent of thepH of the solu'tion to be tested. In case of manganese ion contents ofmore than 500 mg./l., the solution to be tested or an aliquot portionthereof, is first diluted to such an extent that a blue coloration lyingwithin the varying color scale is attained. By multiplication, inaccordance with the selected dilution, a semi-quantitative indication ofthe concentration of the manganese ions is also obtained in this mannerwith solutions of high Mn concentration.

The novel indicator is very specific; the influence of other cations onthe test is extremely minor. Thus, the

detection is not impaired by less than 1,000 mg./l. respectively, of thefollowing ions: Al As, Ba, Be, a+ 2+ 2+ 2+ 2+ 1 3+ 2+ Kt Mg2+NH t 2+ p2+. Ro+. 503+ 2+ 1+ 2+ Zr.

If the thus-produced color of the testing zone deviates from the colorscale, the disturbance may be due to concentration of foreign ions whichare too high. In this case, the interfering ions must be masked byadding to the solution to be tested a small amount of a maskingsubstance, e.g., a spatula-tip amount to about 5 ml. of the solution.

For example, Ti is masked with potassium fluoride, Ag and Hg can beseparated by the addition of NaCl. In the case of very high iron (llI)concentrations, sodium acetate is added to the acidic solution until apH of 4 4.8 is attained. Then, the solution is briefly boiled,filtered,and.the manganese ion concentration in the filtate isdetermined.

The influence of anions on the test is also extremely low. Theindication is not impaired by less than respectively 1,000 mg./l. of Br,BrOf, CN, Cl", C10 .0 01; r-tirprcwtrn 9:1 ew-. 513: OCN, PO scN SeO so,S S 0 S 0 W0 acetate, ascorbate, citrate, oxalate, succinate, tartrate,and 500 mg./l. of S 0 The novel indicator thus exhibts considerableadvantages over the previously known indicating reactions for manganeseions. For this first time, a shelf-stable indicator has been developed,by means of which simiquantitative determinations are possible with avery high degree of selectivity. The indivating accuracy is independentof the pH of the solution to be tested within a very wide pH range andthe testfunctions eve in the presence of larger amounts of iron ions.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever. 1

EXAMPLE 1 Filter paper (No. 604, Schleicher & Schilll) is impregnatedwith solution I, dried, impregnated with solution I! and once againdried. The thus-obtained impregnated filter paper is cut into smallsquares of about 6 X 6 mm. and cemented to 6 X 60 mm plastic strips atthe lower end thereof.

Impregnating solution 1 contains:

1 g. of o-tolidine 100 ml. of methanol.

lmpregnating solution Il contains: 3 g. of magnesium sulfateheptahydrate l g. of tartaric acid 100 ml. of water.

The test strip after being immersed into a solution containing, inaddition to iron (Ill) ions, 10 mg./l. of manganese ions, then bathingthe testing zone first in l-2N sodium hydroxide solution and then inabout 10 percent strength aqueous acetic acid, exhibits a blue coloringaccurately corresponding, upon comparison with the appropriate colorscale, to a value of 10 mg./l. of Mn I0 diaminofluorene,

dinitrobenzidine,

EXAMPLE 2 This example is conducted analogously to Example I, but theimpregnating solution is 0.3 g. of o-tolidine in 5 ml. of 2 percentaqueous acetic acid. impregnating solution II contains 2 g. of magnesiumsulfate heptahydrate and 0.5 g. of tartaric acid in l00 ml. of water.

Analogous results are obtained when in place of otolidine a like amountof benzidine, o-dianisidine, 2,7- 3,3 '-dipropylb'enzidine, 3,3-dipropyloxy-benzidine, 3,3'-dichlorobenzidine or 3,3- dibromobenzidineis used.

Scmiquantitative manganese determination can also be conducted with thisindicator.

EXAMPLE 3 lmpregnating solutions are produced analogously to Example 1,except that impregnating solution 1 contains, in place of o-tolidine, alike amount of 3,3- 3 ,3 '-dicyanobenzidine, 2,7- diaminocarbazole,2,7-diaminodibenzothiophene, or 2,7-diaminodibenzofurane. Theindicator'shows, after immersion into the testing solution described inExampie 1 and subsequent bathing in the solutions set forth 25 therein,a blue to violet coloring, depending on the amine employed. Whencompared with the appropriate color scale, the coloring accuratelyindicates a content of 10 mg./l. of manganese ions.

EXAMPLE 4 The impregnating solutions analogous to Example 2 are combinedunder rapid agitation. A filter paper is 35 impregnated withfthissuspension and then dried.

Semiquantitative manganese determinations can be conducted with thisindicator also.

A .20 g. of pure linters paper is beaten in a solution of .0.2 g. ofo-tolidine in 2 l. of 2 percent acetic acid. Under vigorous'agitation, asolution of 0.3 g. of magnesium sulfate heptahydrate in 50 ml. of wateris added dropwise to the slurry. From this fiber pulp, a paper isproduced on a Fourdrinier paper-making machine. After drying, the papercan additionally be impregnated with a tartaric acid solution.

This paper is also well suited for determining manganese ionconcentration down to 10 p.p.m.

The preceding examples can be repeated with similar successbysubstituting the generically or specifically described reacts and/oroperating conditions of this invention for those used in the precedingexamples. From the foregoing description, one skilled in the art caneasily ascertain the essential characteristics of this invention, andwithout departing from the spirit and scope thereof, can make variouschanges and modifications of the invention to adapt it to various usagesand conditions.

What is claimed is:

1. An indicator for the colorimetric detection of manganese (ll) ions,comprising an absorbent support uniformly impregnated with asubstantially insoluble sulfate salt of an aromatic amine oxidizable bymanganese dioxide to a colored oxidation product and represented by thegeneral formula wherein the radicals R, which can be the same ordifferent, are H, lower alkoxy of one to four carbon atoms, lower alkylof one to four carbon atoms, halogen, nitro, or cyano, and the radicalsR are each H, or together CH -NH-, S, or O-.

2. An indicator according to claim 1 impregnated with an iron ioncomplex-forming agent.

3. An indicator according to claim 2 wherein the complex-forming agentis tartaric or citric acid.

4. The indicator according to claim 1, wherein the aromatic amine iso-tolidine, benzidine, o-dianisidine or 2,7-diaminofluorene.

5. An indicator according to claim 4, impregnated with an iron ioncomplex-forming agent.

6. An indicator according to claim 5, wherein the complex-forming agentis tartaric or citric acid.

7. An indicator according to claim 6 wherein the amine is o-tolidine 8.A process for preparing an indicator according to claim 1, whichcomprises impregnating the absorbing support with one or moreimpregnating solutions collectively containing a 0.05 1 percent solutionof an aromatic amine and a 0.05 4 percent solution of one or both of awatersoluble alkali sulfate and an alkaline earth sulfate.

9. A process according to claim 8 wherein one of the impregnatingsolutions contains 0.2 percent of organic complexing agent, capable offorming complexes with iron ions.

10. A process according to claim 9 wherein the organic complexing agentis tartaric acid, citric acid or a salt thereof.

11. A process according to claim 9, wherein the adsorbent carrier isfirst impregnated with a solution of the organic amine, dried andthereafter impregnated with a solution of the metal sulfate.

12. A process according to claim 1 1 wherein the aromatic amine isdissolved in an organic solvent or dilute acetic acid.

13. A process according to claim 12, wherein a water-soluble metal saltis in an aqueous solution.

14. A process according to claim 13 wherein the metal sulfate is one ormore of sodium, potassium, and magnesium sulfate.

15. A process for the colorimetric determination of manganese ions inaqueous solutions which comprises moistening an indicator of claim 1with the aqueous solution to be tested.

16. A process for the colorimetric determination of manganese ions inaqueous solutions which comprises moistening an indicator of claim 2with the aqueous solution to be tested.

17. A process for the colorimetric determination of manganese ions inaqueous solutions which comprises moistening an indicator of claim 3with the aqueous solution to be tested.

18. A process for the colorimetric determination of manganese ions inaqueous solutions which comprises moistening an indicator of claim 4with the aqueous solution to be tested.

19. A process for the colorimetric determination of manganese ions inaqueous solutions which comprises moistening an indicator of claim 5with the aqueous solution to be tested.

20. A process for the colorimetric determination of manganese ions inaqueous solutions which comprises moistening an indicator of claim 6with the aqueous solution to be tested.

2. An indicator according to claim 1 impregnated with an iron ioncomplex-forming agent.
 3. An indicator according to claim 2 wherein thecomplex-forming agent is tartaric or citric acid.
 4. The indicatoraccording to claim 1, wherein the aromatic amine is o-tolidine,benzidine, o-dianisidine or 2,7-diaminofluorene.
 5. An indicatoraccording to claim 4, impregnated with an iron ion complex-formingagent.
 6. An indicator according to claim 5, wherein the complex-formingagent is tartaric or citric acid.
 7. An indicator according to claim 6wherein the amine is o-tolidine
 8. A process for preparing an indicatoraccording to claim 1, which comprises impregnating the absorbing supportwith one or more impregnating solutions collectively containing a 0.05 -1 percent solution of an aromatic amine and a 0.05 - 4 percent solutionof one or both of a water-soluble alkali sulfate and an alkaline earthsulfate.
 9. A process according to claim 8 wherein one of theimpregnating solutions contains 0.2 - 5 percent of organic complexingagent, capable of forming complexes with iron ions.
 10. A processaccording to claim 9 wherein the organic complexing agent is tartaricacid, citric acid or a salt thereof.
 11. A process according to claim 9,wherein the adsorbent carrier is first impregnated with a solution ofthe organic amine, dried and thereafter impregnated with a solution ofthe metal sulfate.
 12. A process according to claim 11 wherein thearomatic amine is dissolved in an organic solvent or dilute acetic acid.13. A process according to claim 12, wherein a water-soluble metal saltis in an aqueous solution.
 14. A process according to claim 13 whereinthe metal sulfate is one or more of sodium, potassium, and magnesiumsulfate.
 15. A process for the colorimetric determination of manganeseions in aqueous solutions which comprises moistening an indicator ofclaim 1 with the aqueous solution to be tested.
 16. A process for thecolorimetric determination of manganese ions in aqueous solutions whichcomprises moistening an indicator of claim 2 with the aqueous solutionto be tested.
 17. A process for the colorimetric determination ofmanganese ions in aqueous soluTions which comprises moistening anindicator of claim 3 with the aqueous solution to be tested.
 18. Aprocess for the colorimetric determination of manganese ions in aqueoussolutions which comprises moistening an indicator of claim 4 with theaqueous solution to be tested.
 19. A process for the colorimetricdetermination of manganese ions in aqueous solutions which comprisesmoistening an indicator of claim 5 with the aqueous solution to betested.
 20. A process for the colorimetric determination of manganeseions in aqueous solutions which comprises moistening an indicator ofclaim 6 with the aqueous solution to be tested.